Etch process for reducing silicon recess

ABSTRACT

A method for selectively etching a substrate is described. The method includes disposing a substrate comprising a silicon nitride (SiN y ) layer overlying silicon in a plasma etching system, and transferring a pattern to the silicon nitride layer using a plasma etch process, wherein the plasma etch process utilizes a process composition having as incipient ingredients a process gas containing C, H and F, and an additive gas including CO 2 . The method further includes: selecting an amount of the additive gas in the plasma etch process to achieve: (1) a silicon recess formed in the silicon having a depth less than 10 nanometers (nm), and (2) a sidewall profile in the pattern having an angular deviation from 90 degrees less than 2 degrees.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention is related to U.S. patent application Ser. No.11/226,452, entitled “METHOD AND SYSTEM FOR ETCHING SILICON OXIDE ANDSILICON NITRIDE WITH HIGH SELECTIVITY RELATIVE TO SILICON”, filed onSep. 15, 2005, and now abandoned; and U.S. patent application Ser. No.11/350,765, entitled “METHOD AND SYSTEM FOR SELECTIVELY ETCHING ADIELECTRIC MATERIAL RELATIVE TO SILICON”, filed on Feb. 10, 2006, andnow issued as U.S. Pat. No. 7,393,788; the entire contents of which areherein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to a method and system for selectivelyetching dielectric materials, and more particularly to a method andsystem for uniformly etching silicon oxide (SiO_(x)) or silicon nitride(SiN_(y)) with high selectivity relative to silicon using a plasma etchprocess that achieves an anisotropic etch profile with reduced siliconrecess.

2. Description of Related Art

Typically, during fabrication of integrated circuits (ICs),semiconductor production equipment utilize a (dry) plasma etch processto remove or etch material along fine lines or within vias or contactspatterned on a semiconductor substrate. The success of the plasma etchprocess requires that the etch chemistry includes chemical reactantssuitable for selectively etching one material while substantially notetching another material.

For example, on a semiconductor substrate, a pattern formed in aprotective layer can be transferred to an underlying layer of a selectedmaterial utilizing a plasma etching process. The protective layer cancomprise a light-sensitive layer, such as a photoresist layer, having apattern formed using a lithographic process.

Once the pattern is formed, the semiconductor substrate is disposedwithin a plasma processing chamber, and an etching chemistry is formedthat selectively etches the underlying layer while minimally etching theprotective layer. This etch chemistry is produced by introducing anionizable, dissociative gas mixture having parent molecules comprisingmolecular constituents capable of reacting with the underlying layerwhile minimally reacting with the protective layer. The production ofthe etch chemistry comprises introduction of the gas mixture andformation of plasma when a portion of the gas species present areionized following a collision with an energetic electron. Moreover, theheated electrons serve to dissociate some species of the gas mixture andcreate a reactive mixture of chemical constituents (of the parentmolecules).

Thereafter, the ionized gas species and reactive mixture of chemicalconstituents facilitate the etching of various features (e.g., trenches,vias, contacts, etc.) in the exposed regions of substrate. Suchsubstrate materials where etching is required include silicon dioxide(SiO₂), silicon nitride (Si₃N₄), poly-crystalline silicon (polysilicon),and mono-crystalline silicon (silicon), for example.

SUMMARY OF THE INVENTION

The present invention relates to a method and system for selectivelyetching dielectric materials, and more particularly to a method andsystem for uniformly etching silicon oxide (SiO_(x)) or silicon nitride(SiN_(y)) with high selectivity relative to silicon using a plasma etchprocess that achieves an anisotropic etch profile with reduced siliconrecess.

According to one embodiment, a method for selectively etching asubstrate is described. The method includes disposing a substratecomprising a silicon nitride (SiN_(y)) layer overlying silicon in aplasma etching system, and transferring a pattern to the silicon nitridelayer using a plasma etch process, wherein the plasma etch processutilizes a process gas composition having as incipient ingredients aprocess gas containing C, H and F, and an additive gas including CO₂.The method further includes: selecting an amount of the additive gas inthe plasma etch process to achieve: (1) a silicon recess formed in thesilicon having a depth less than 10 nanometers (nm), and (2) a sidewallprofile in the pattern having an angular deviation from 90 degrees lessthan 2 degrees.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B show a schematic representation of a structure formed ona silicon substrate;

FIG. 2 provides a flow chart illustrating a method of selectivelyetching a substrate according to an embodiment;

FIG. 3 provides a schematic illustration of a pattern formed in a layeron a substrate according to another embodiment;

FIG. 4 shows a schematic representation of a plasma etching systemaccording to an embodiment;

FIG. 5 shows a schematic representation of a plasma etching systemaccording to another embodiment;

FIG. 6 shows a schematic representation of a plasma etching systemaccording to another embodiment;

FIG. 7 shows a schematic representation of a plasma etching systemaccording to another embodiment;

FIG. 8 shows a schematic representation of a plasma etching systemaccording to another embodiment;

FIG. 9 shows a schematic representation of a plasma etching systemaccording to another embodiment;

FIG. 10 shows a schematic representation of a plasma etching systemaccording to another embodiment;

FIG. 11 shows a schematic representation of a plasma etching systemaccording to another embodiment;

FIG. 12 shows a schematic representation of a substrate holder for usein a plasma etching system according to yet another embodiment;

FIG. 13 provides a schematic illustration of a film stack according toanother embodiment;

FIGS. 14A through 14D provide a SEM photograph of a patterning resultusing a plasma etch process;

FIGS. 15A and 15B provide a SEM photograph of a patterning result usinga plasma etch process according to an embodiment; and

FIGS. 16A and 16B provide a SEM photograph of a patterning result usinga plasma etch process according to yet another embodiment.

DETAILED DESCRIPTION OF SEVERAL EMBODIMENTS

In the following description, for purposes of explanation and notlimitation, specific details are set forth, such as a particulargeometry of a processing system, descriptions of various components andprocesses used therein. However, it should be understood that theinvention may be practiced in other embodiments that depart from thesespecific details.

Similarly, for purposes of explanation, specific numbers, materials, andconfigurations are set forth in order to provide a thoroughunderstanding of the invention. Nevertheless, the invention may bepracticed without specific details. Furthermore, it is understood thatthe various embodiments shown in the figures are illustrativerepresentations and are not necessarily drawn to scale.

Various operations will be described as multiple discrete operations inturn, in a manner that is most helpful in understanding the invention.However, the order of description should not be construed as to implythat these operations are necessarily order dependent. In particular,these operations need not be performed in the order of presentation.Operations described may be performed in a different order than thedescribed embodiment. Various additional operations may be performedand/or described operations may be omitted in additional embodiments.

“Substrate” as used herein generically refers to the object beingprocessed in accordance with the invention. The substrate may includeany material portion or structure of a device, particularly asemiconductor or other electronics device, and may, for example, be abase substrate structure, such as a semiconductor wafer or a layer on oroverlying a base substrate structure such as a thin film. Thus,substrate is not intended to be limited to any particular basestructure, underlying layer or overlying layer, patterned orun-patterned, but rather, is contemplated to include any such layer orbase structure, and any combination of layers and/or base structures.The description below may reference particular types of substrates, butthis is for illustrative purposes only and not limitation.

In material processing methodologies, dry plasma etching utilizes aplasma chemistry having chemical reactants suitable for selectivelyetching one material while substantially not etching another material.In one example, a layer of insulating (dielectric) material is depositedover a gate stack having a poly-crystalline silicon (polysilicon)feature, see FIG. 1A. For example, the insulating layer may comprisesilicon oxide (e.g., SiO_(x)), or silicon nitride (e.g., SiN_(y)), orboth.

Thereafter, the insulating layer is subjected to a plasma etch process,wherein the insulating layer is removed in all locations except alongthe sidewalls of the gate stack; see FIG. 1B. The remaining insulatingmaterial acts as a spacer in the fabrication of the semiconductordevice. It is important for device operation and/or reliability that thespacer be formed without substantially reducing the polysilicon gatematerial and while minimizing a recess (FIG. 1B) formed in the siliconsubstrate. Furthermore, it is important to meet the aforementionedconditions while achieving an anisotropic profile.

Thus, an etch chemistry is preferably chosen to etch the insulatingmaterial while minimally etching the underlying (mono-crystalline)silicon substrate, as well as minimally etching the polysilicon.Furthermore, it is important for manufacturing yields, for example, thatthe results of the spacer etch process are uniform across the extent ofthe substrate. As described above, the plasma etch process must performaccording to rigid specifications to achieve properly dimensioned,robust electrical structures in the IC. Other applications are alsocontemplated including contact align and dual stress liner formation.

Accordingly, in one embodiment a method and system for selectively anduniformly etching silicon oxide (SiO_(x)) and/or silicon nitride(SiN_(y)) with respect to silicon and polysilicon in a plasma etchingsystem are described. As illustrated in FIGS. 2 and 3, the methodincludes a flow chart 100 beginning in 110 with disposing a substrate210 comprising a silicon nitride (SiN_(y)) layer 220 overlying siliconin a plasma etching system. The substrate 210 further comprises apatterned mask layer 230 having one or more layers (e.g., lithographicmask layer, soft mask layer, hard mask layer, anti-reflective coating(ARC), organic planarization layer (OPL), etc.).

In 120 and as shown in FIG. 3, a pattern 240 is transferred to thesilicon nitride layer 220 using a plasma etch process, wherein theplasma etch process utilizes a process gas composition having asincipient ingredients a process gas containing C, H and F, and anadditive gas including CO₂. As an example, the process gas containing C,H and F may include CHF₃, CH₃F, CH₂F₂, or any combination of two or morethereof. As another example, the process gas containing C, H and F mayinclude CHF₃. As another example, the process gas containing C, H and Fmay consist of CHF₃. The process gas composition may further include anoble gas, such as He, Ne, Ar, Kr, or Xe. Additionally, the process gascomposition may further include an oxygen-containing gas, such as oxygen(O₂), CO, NO, N₂O, or NO₂, or any combination thereof. As yet anotherexample, the process gas composition may comprise CHF₃, CO₂, and Ar. Aseven yet another example, the process gas composition may consist ofCHF₃, CO₂, and Ar.

Furthermore, in 130 and as shown in FIG. 3, an amount of the additivegas in the plasma etch process is selected to achieve: (1) a siliconrecess 242 formed in the silicon having a depth less than 10 nanometers(nm), and (2) a sidewall profile in the pattern 240 having an angulardeviation 244 from 90 degrees less than 2 degrees. Alternatively, thesilicon recess 242 formed in the silicon has a depth less than 8nanometers (nm), and the angular deviation 244 from 90 degrees is lessthan 1 degree.

As described above, the process gas composition comprises use of a gascollectively containing C, H and F, or fluorohydrocarbons, such as CH₂F₂and CHF₃. High etch selectivity and acceptable uniformity can beachieved by selecting a process condition, including a pressure, a flowrate of CHF₃, and a radio frequency (RF) power coupled to the plasmaetching system, such that a proper balance of active etching moleculesor atoms, and polymer forming molecules are formed within the etchingplasma.

For example, it is believed by the inventors that the use of an etchchemistry collectively containing C, H and F promotes the formation ofhydrocarbon and fluorocarbon molecules in the presence of the etchingplasma that may adsorb on polysilicon and silicon surfaces and protectthese surfaces during the etching process, while permitting the etchingof silicon oxide or silicon nitride surfaces. An etch chemistrycollectively containing C, H and F may produce a balance of activelyetching molecules or atoms and polymer forming molecules. The protectivenature of this chemistry; however, may be detrimental to the etchprofile and degrade the anisotropy of the plasma etch process.Therefore, the additive gas including CO₂ is introduced to improveanisotropy, while maintaining reduced silicon recess formation (to bediscussed below). The inventors explored oxygen (O₂); however, O₂addition was found to substantially degrade the protective nature of theetch chemistry and cause unacceptable silicon recess.

To achieve an anisotropic etch profile with reduced silicon recess, aflow rate of the process gas containing C, H and F is selected to rangefrom about 10 sccm (standard cubic centimeters per minute) to about 50sccm, and a flow rate of the additive gas is selected to range fromabout 1 sccm to about 10 sccm. The pressure in the plasma etching systemis selected to range from 1 mTorr (milli-Torr) to 200 mTorr, e.g., 30mTorr to 60 mTorr.

In one embodiment, a ratio between a flow rate of the additive gas and aflow rate of the process gas containing C, H and F is selected to rangefrom 5% to 25%. In another embodiment, a ratio between a flow rate ofthe additive gas and a flow rate of the process gas containing C, H andF is selected to range from 10% to 20%. In another embodiment, a ratiobetween a flow rate of the additive gas and a flow rate of the processgas containing C, H and F is selected to range from 15% to 20%. In yetanother embodiment, a ratio between a flow rate of the CO₂ and a flowrate of CHF₃ is selected to range from 15% to 20%.

As will be described in greater detail below, the substrate temperaturemay be controlled according to a temperature control scheme using atemperature controlled substrate holder in the plasma etching system.Using the temperature controlled substrate holder in the plasma etchingsystem, the substrate temperature may be spatially and temporallycontrolled to improve the plasma etch process.

The temperature controlled substrate holder may comprise a support basehaving fluid channels to circulate a temperature controlled thermalfluid in the support base, and a substrate support coupled via a thermalinsulator to an upper portion of the support base. The substrate supportfurther comprises one or more heating elements embedded within thesubstrate support, an upper surface to support the substrate by contactbetween the upper surface and a backside of the substrate, and anelectrostatic clamp electrode to hold the substrate on the upper surfaceof the substrate support. The one or more heating elements may comprisea first heating element located at a substantially central region of thesubstrate and a second heating element located at a substantially edgeregion of the substrate, wherein the first heating element and thesecond heating element are concentrically arranged.

Additionally, the temperature controlled substrate holder may include abackside gas supply system configured to supply a heat transfer gas tothe backside of the substrate through at least one of a plurality oforifices or channels disposed on the upper surface of the substratesupport. The orifices of the backside gas supply system may be arrangedin a plurality of zones on the upper surface of the substrate support tovary a backside pressure in a radial direction between a substantiallycentral region of the backside of the substrate and a substantially edgeregion of the backside of the substrate. For example, the plurality ofzones for controlling the supply of heat transfer gas to the backside ofthe substrate may correspond to the regions where the first and secondheating elements are located.

The process pressure may be varied during the plasma etch process.Additionally, during the plasma etch process, power for generatingplasma may be varied, or it may be kept constant. Furthermore, theplasma etch process may proceed for a time duration sufficient to etchpartially or fully through the silicon nitride layer 220 in the filmstack. The time duration may be determined in-situ using endpointdetection or it may be determined prior to performing each plasma etchprocess. To address etch uniformity, etch profile control, and/orcritical dimension (CD) control, the duration of the plasma etch processmay be extended by an over-etch process.

The plasma etch process described above may be performed utilizing aplasma etching system such as the one described in FIGS. 4 through 11.Furthermore, the plasma etch process described above may be performedutilizing a temperature controlled substrate holder in a plasma etchingsystem such as the one described in FIG. 12.

According to one embodiment, a plasma etching system 1 is depicted inFIG. 4 comprising a plasma processing chamber 2, a diagnostic system 3coupled to the plasma processing chamber 2, and a controller 4 coupledto the diagnostic system 3 and the plasma processing chamber 2.

The controller 4 is configured to execute a process recipe comprising aprocess gas composition having as incipient ingredients a process gascontaining C, H and F, and an additive gas including CO₂ to selectivelyand uniformly etch silicon oxide or silicon nitride relative to siliconand/or polysilicon. Alternatively, the controller 4 is configured toexecute a process recipe comprising trifluoromethane (CHF₃), carbondioxide (CO₂), and a noble gas to selectively etch silicon oxide orsilicon nitride relative to silicon and/or polysilicon. Additionally,controller 4 is configured to receive at least one endpoint signal fromthe diagnostic system 3 and to post-process the at least one endpointsignal in order to accurately determine an endpoint for the plasma etchprocess. In the illustrated embodiment, plasma etching system 1,depicted in FIG. 4, utilizes plasma for material processing.

According to another embodiment, a plasma etching system 1 a configuredto perform the above identified process conditions is depicted in FIG. 5comprising a plasma processing chamber 10, substrate holder 20, uponwhich a substrate 25 to be processed is affixed, and vacuum pumpingsystem 50. Substrate 25 can be a semiconductor substrate, a wafer, aflat panel display, or a liquid crystal display. Plasma processingchamber 10 can be configured to facilitate the generation of plasma inprocessing region 45 in the vicinity of a surface of substrate 25. Anionizable gas or mixture of process gases is introduced via a gasdistribution system 40. For a given flow of process gas, the processpressure is adjusted using the vacuum pumping system 50. Plasma can beutilized to create materials specific to a pre-determined materialsprocess, and/or to aid the removal of material from the exposed surfacesof substrate 25. The plasma etching system 1 a can be configured toprocess substrates of any desired size, such as 200 mm substrates, 300mm substrates, or larger.

Substrate 25 can be affixed to the substrate holder 20 via a clampingsystem 28, such as a mechanical clamping system or an electricalclamping system (e.g., an electrostatic clamping system). Furthermore,substrate holder 20 can include a heating system (not shown) or acooling system (not shown) that is configured to adjust and/or controlthe temperature of substrate holder 20 and substrate 25. The heatingsystem or cooling system may comprise a re-circulating flow of heattransfer fluid that receives heat from substrate holder 20 and transfersheat to a heat exchanger system (not shown) when cooling, or transfersheat from the heat exchanger system to substrate holder 20 when heating.In other embodiments, heating/cooling elements, such as resistiveheating elements, or thermo-electric heaters/coolers can be included inthe substrate holder 20, as well as the chamber wall of the plasmaprocessing chamber 10 and any other component within the plasma etchingsystem 1 a.

Additionally, a heat transfer gas can be delivered to the backside ofsubstrate 25 via a backside gas supply system 26 in order to improve thegas-gap thermal conductance between substrate 25 and substrate holder20. Such a system can be utilized when temperature control of thesubstrate is required at elevated or reduced temperatures. For example,the backside gas supply system can comprise a two-zone gas distributionsystem, wherein the helium gas-gap pressure can be independently variedbetween the center and the edge of substrate 25.

In the embodiment shown in FIG. 5, substrate holder 20 can comprise anelectrode 22 through which RF power is coupled to the processing plasmain processing region 45. For example, substrate holder 20 can beelectrically biased at a RF voltage via the transmission of RF powerfrom a RF generator 30 through an optional impedance match network 32 tosubstrate holder 20. The RF bias can serve to heat electrons to form andmaintain plasma. In this configuration, the system can operate as areactive ion etch (RIE) reactor, wherein the chamber and an upper gasinjection electrode serve as ground surfaces. A typical frequency forthe RF bias can range from about 0.1 MHz to about 100 MHz. RF systemsfor plasma processing are well known to those skilled in the art.

Alternately, RF power is applied to the substrate holder electrode atmultiple frequencies. Furthermore, impedance match network 32 canimprove the transfer of RF power to plasma in plasma processing chamber10 by reducing the reflected power. Match network topologies (e.g.L-type, π-type, T-type, etc.) and automatic control methods are wellknown to those skilled in the art.

Gas distribution system 40 may comprise a showerhead design forintroducing a mixture of process gases. Alternatively, gas distributionsystem 40 may comprise a multi-zone showerhead design for introducing amixture of process gases and adjusting the distribution of the mixtureof process gases above substrate 25. For example, the multi-zoneshowerhead design may be configured to adjust the process gas flow orcomposition to a substantially peripheral region above substrate 25relative to the amount of process gas flow or composition to asubstantially central region above substrate 25.

Vacuum pumping system 50 can include a turbo-molecular vacuum pump (TMP)capable of a pumping speed up to about 5000 liters per second (andgreater) and a gate valve for throttling the chamber pressure. Inconventional plasma processing devices utilized for dry plasma etching,a 1000 to 3000 liter per second TMP can be employed. TMPs are useful forlow pressure processing, typically less than about 50 mTorr. For highpressure processing (i.e., greater than about 100 mTorr), a mechanicalbooster pump and dry roughing pump can be used. Furthermore, a devicefor monitoring chamber pressure (not shown) can be coupled to the plasmaprocessing chamber 10.

Controller 55 comprises a microprocessor, memory, and a digital I/O portcapable of generating control voltages sufficient to communicate andactivate inputs to plasma etching system 1 a as well as monitor outputsfrom plasma etching system 1 a. Moreover, controller 55 can be coupledto and can exchange information with RF generator 30, impedance matchnetwork 32, the gas distribution system 40, vacuum pumping system 50, aswell as the substrate heating/cooling system (not shown), the backsidegas delivery system 26, and/or the electrostatic clamping system 28. Forexample, a program stored in the memory can be utilized to activate theinputs to the aforementioned components of plasma etching system 1 aaccording to a process recipe in order to perform a plasma assistedprocess on substrate 25.

Controller 55 can be locally located relative to the plasma etchingsystem 1 a, or it can be remotely located relative to the plasma etchingsystem 1 a. For example, controller 55 can exchange data with plasmaetching system 1 a using a direct connection, an intranet, and/or theinternet. Controller 55 can be coupled to an intranet at, for example, acustomer site (i.e., a device maker, etc.), or it can be coupled to anintranet at, for example, a vendor site (i.e., an equipmentmanufacturer). Alternatively or additionally, controller 55 can becoupled to the internet. Furthermore, another computer (i.e.,controller, server, etc.) can access controller 55 to exchange data viaa direct connection, an intranet, and/or the internet.

In the embodiment shown in FIG. 6, plasma etching system 1 b can besimilar to the embodiment of FIG. 5 and further comprise either astationary, or mechanically or electrically rotating magnetic fieldsystem 60, in order to potentially increase plasma density and/orimprove plasma processing uniformity, in addition to those componentsdescribed with reference to FIG. 5. Moreover, controller 55 can becoupled to magnetic field system 60 in order to regulate the speed ofrotation and field strength. The design and implementation of a rotatingmagnetic field is well known to those skilled in the art.

In the embodiment shown in FIG. 7, plasma etching system 1 c can besimilar to the embodiment of FIG. 5 or FIG. 6, and can further comprisean upper electrode 70 to which RF power can be coupled from RF generator72 through optional impedance match network 74. A frequency for theapplication of RF power to the upper electrode can range from about 0.1MHz to about 200 MHz. Additionally, a frequency for the application ofpower to the lower electrode can range from about 0.1 MHz to about 100MHz. Moreover, controller 55 is coupled to RF generator 72 and impedancematch network 74 in order to control the application of RF power toupper electrode 70. The design and implementation of an upper electrodeis well known to those skilled in the art. The upper electrode 70 andthe gas distribution system 40 can be designed within the same chamberassembly, as shown.

In the embodiment shown in FIG. 8, plasma etching system 1 c′ can besimilar to the embodiment of FIG. 7, and can further comprise a directcurrent (DC) power supply 90 coupled to the upper electrode 70 opposingsubstrate 25. The upper electrode 70 may comprise an electrode plate.The electrode plate may comprise a silicon-containing electrode plate.Moreover, the electrode plate may comprise a doped silicon electrodeplate. The DC power supply 90 can include a variable DC power supply.Additionally, the DC power supply can include a bipolar DC power supply.The DC power supply 90 can further include a system configured toperform at least one of monitoring, adjusting, or controlling thepolarity, current, voltage, or on/off state of the DC power supply 90.Once plasma is formed, the DC power supply 90 facilitates the formationof a ballistic electron beam. An electrical filter (not shown) may beutilized to de-couple RF power from the DC power supply 90.

For example, the DC voltage applied to upper electrode 70 by DC powersupply 90 may range from approximately −2000 volts (V) to approximately1000 V. Desirably, the absolute value of the DC voltage has a valueequal to or greater than approximately 100 V, and more desirably, theabsolute value of the DC voltage has a value equal to or greater thanapproximately 500 V. Additionally, it is desirable that the DC voltagehas a negative polarity. Furthermore, it is desirable that the DCvoltage is a negative voltage having an absolute value greater than theself-bias voltage generated on a surface of the upper electrode 70. Thesurface of the upper electrode 70 facing the substrate holder 20 may becomprised of a silicon-containing material.

In the embodiment shown in FIG. 9, plasma etching system 1 d can besimilar to the embodiments of FIGS. 5 and 6, and can further comprise aninductive coil 80 to which RF power is coupled via RF generator 82through optional impedance match network 84. RF power is inductivelycoupled from inductive coil 80 through a dielectric window (not shown)to plasma processing region 45. A frequency for the application of RFpower to the inductive coil 80 can range from about 10 MHz to about 100MHz. Similarly, a frequency for the application of power to the chuckelectrode can range from about 0.1 MHz to about 100 MHz. In addition, aslotted Faraday shield (not shown) can be employed to reduce capacitivecoupling between the inductive coil 80 and plasma in the processingregion 45. Moreover, controller 55 can be coupled to RF generator 82 andimpedance match network 84 in order to control the application of powerto inductive coil 80.

In an alternate embodiment, as shown in FIG. 10, plasma etching system 1e can be similar to the embodiment of FIG. 9, and can further comprisean inductive coil 80′ that is a “spiral” coil or “pancake” coil incommunication with the plasma processing region 45 from above as in atransformer coupled plasma (TCP) reactor. The design and implementationof an inductively coupled plasma (ICP) source, or transformer coupledplasma (TCP) source, is well known to those skilled in the art.

Alternately, plasma can be formed using electron cyclotron resonance(ECR). In yet another embodiment, the plasma is formed from thelaunching of a Helicon wave. In yet another embodiment, the plasma isformed from a propagating surface wave. Each plasma source describedabove is well known to those skilled in the art.

In the embodiment shown in FIG. 11, plasma etching system if can besimilar to the embodiment of FIG. 5, and can further comprise a surfacewave plasma (SWP) source 80″. The SWP source 80″ can comprise a slotantenna, such as a radial line slot antenna (RLSA), to which microwavepower is coupled via microwave generator 82′ through optional impedancematch network 84′.

Referring now to FIG. 12, a temperature controlled substrate holder 500for use in any one of the plasma etching systems depicted in FIGS. 4through 11 is described according to yet another embodiment. Thesubstrate holder 500 comprises a substrate support 530 having a firsttemperature and configured to support a substrate 510, atemperature-controlled support base 520 positioned below substratesupport 530 and configured to be at a second temperature less than thefirst temperature (e.g. less than a desired temperature of substrate510), and a thermal insulator 540 disposed between the substrate support530 and the temperature-controlled support base 520. Additionally, thesubstrate support 530 comprises a center heating element 533 (located ata substantially center region below substrate 510) and an edge heatingelement 531 (located at a substantially edge, or peripheral, regionbelow substrate 510) coupled thereto, and configured to elevate thetemperature of the substrate support 530. Furthermore, the support base520 comprises one or more cooling elements 521 coupled thereto, andconfigured to reduce the temperature of the substrate support 530 viathe removal of heat from the substrate support 530 through thermalinsulator 540.

As shown in FIG. 12, the center heating element 533 and the edge heatingelement 531 are coupled to a heating element control unit 532. Heatingelement control unit 532 is configured to provide either dependent orindependent control of each heating element, and exchange informationwith a controller 550. The center heating element 533 and the edgeheating element 531 may comprise at least one of a heating fluidchannel, a resistive heating element, or a thermo-electric elementbiased to transfer heat towards the wafer.

For example, the center heating element 533 and the edge heating element531 may comprise one or more heating channels that can permit flow of afluid, such as water, FLUORINERT, GALDEN HT-135, etc., there through inorder to provide conductive-convective heating, wherein the fluidtemperature has been elevated via a heat exchanger. The fluid flow rateand fluid temperature can, for example, be set, monitored, adjusted, andcontrolled by the heating element control unit 532.

Alternatively, for example, the center heating element 533 and the edgeheating element 531 may comprise one or more resistive heating elementssuch as a tungsten, nickel-chromium alloy, aluminum-iron alloy, aluminumnitride, etc., filament. Examples of commercially available materials tofabricate resistive heating elements include Kanthal, Nikrothal,Akrothal, which are registered trademark names for metal alloys producedby Kanthal Corporation of Bethel, Conn. The Kanthal family includesferritic alloys (FeCrAl) and the Nikrothal family includes austeniticalloys (NiCr, NiCrFe). For example, the heating elements can comprise acast-in heater commercially available from Watlow (1310 Kingsland Dr.,Batavia, Ill., 60510) capable of a maximum operating temperature of 400to 450 degrees C., or a film heater comprising aluminum nitridematerials that is also commercially available from Watlow and capable ofoperating temperatures as high as 300 C and power densities of up to23.25 W/cm². Additionally, for example, the heating element can comprisea silicone rubber heater (1.0 mm thick) capable of 1400 W (or powerdensity of 5 W/in²). When an electrical current flows through thefilament, power is dissipated as heat, and, therefore, the heatingelement control unit 532 can, for example, comprise a controllable DCpower supply. A further heater option, suitable for lower temperaturesand power densities, are Kapton heaters, consisting of a filamentembedded in a Kapton (e.g. polyimide) sheet, marketed by Minco, Inc., ofMinneapolis, Minn.

Alternately, for example, the center heating element 533 and the edgeheating element 531 can comprise an array of thermo-electric elementscapable of heating or cooling a substrate depending upon the directionof electrical current flow through the respective elements. Thus, whilethe center heating element 533 and the edge heating element 531 arereferred to as “heating elements,” these elements may include thecapability of cooling in order to provide rapid transition betweentemperatures. Further, heating and cooling functions may be provided byseparate elements within the substrate support 530. An exemplarythermo-electric element is one commercially available from AdvancedThermoelectric, Model ST-127-1.4-8.5M (a 40 mm by 40 mm by 3.4 mmthermo-electric device capable of a maximum heat transfer power of 72W). Therefore, the heating element control unit 532 can, for example,comprise a controllable current source.

The one or more cooling elements 521 can comprise at least one of acooling channel, or a thermo-electric element. Furthermore, as shown inFIG. 12, the one or more cooling elements 521 are coupled to a coolingelement control unit 522. Cooling element control unit 522 is configuredto provide dependent or independent control of each cooling element 521,and exchange information with controller 550.

For example, the one or more cooling elements 521 can comprise one ormore cooling channels that can permit flow of a fluid, such as water,FLUORINERT, GALDEN HT-135, etc., there through in order to provideconductive-convective cooling, wherein the fluid temperature has beenlowered via a heat exchanger. The fluid flow rate and fluid temperaturecan, for example, be set, monitored, adjusted, and controlled by thecooling element control unit 522. Alternately, during heating forexample, the fluid temperature of the fluid flow through the one or morecooling elements 521 may be increased to complement the heating by thecenter heating element 533 and the edge heating element 531. Alternatelyyet, during cooling for example, the fluid temperature of the fluid flowthrough the one or more cooling elements 521 may be decreased.

Alternately, for example, the one or more cooling elements 521 cancomprise an array of thermo-electric elements capable of heating orcooling a substrate depending upon the direction of electrical currentflow through the respective elements. Thus, while the elements 521 arereferred to as “cooling elements,” these elements may include thecapability of heating in order to provide rapid transition betweentemperatures. Further, heating and cooling function may be provided byseparate elements within the temperature controlled support base 520. Anexemplary thermo-electric element is one commercially available fromAdvanced Thermoelectric, Model ST-127-1.4-8.5M (a 40 mm by 40 mm by 3.4mm thermo-electric device capable of a maximum heat transfer power of 72W). Therefore, the cooling element control unit 522 can, for example,comprise a controllable current source.

Additionally, as shown in FIG. 12, the substrate holder 500 may furthercomprise an electrostatic clamp (ESC) comprising one or more clampingelectrodes 535 embedded within substrate support 530. The ESC furthercomprises a high-voltage (HV) DC voltage supply 534 coupled to theclamping electrodes 535 via an electrical connection. The design andimplementation of such a clamp is well known to those skilled in the artof electrostatic clamping systems. Furthermore, the HV DC voltage supply534 is coupled to controller 550 and is configured to exchangeinformation with controller 550.

Furthermore, as shown in FIG. 12, the substrate holder 500 can furthercomprise a back-side gas supply system 536 for supplying a heat transfergas, such as an inert gas including helium, argon, xenon, krypton, aprocess gas, or other gas including oxygen, nitrogen, or hydrogen, tothe center region and the edge region of the backside of substrate 510through two gas supply lines, and at least two of a plurality oforifices and channels (not shown). The backside gas supply system 536,as shown, comprises a two-zone (center/edge) system, wherein thebackside pressure can be varied in a radial direction from the center toedge. Furthermore, the backside gas supply system 536 is coupled tocontroller 550 and is configured to exchange information with controller550.

Further yet, as shown in FIG. 12, the substrate holder 500 furthercomprises a center temperature sensor 562 for measuring a temperature ata substantially center region below substrate 510 and an edgetemperature sensor 564 for measuring a temperature at a substantiallyedge region below substrate 510. The center and edge temperature sensors562, 564 are coupled to a temperature monitoring system 560.

The temperature sensor can include an optical fiber thermometer, anoptical pyrometer, a band-edge temperature measurement system asdescribed in U.S. Pat. No. 6,891,124, the contents of which areincorporated herein by reference in their entirety, or a thermocouple(as indicated by the dashed line) such as a K-type thermocouple.Examples of optical thermometers include: an optical fiber thermometercommercially available from Advanced Energies, Inc., Model No. OR2000F;an optical fiber thermometer commercially available from LuxtronCorporation, Model No. M600; or an optical fiber thermometercommercially available from Takaoka Electric Mfg., Model No. FT-1420.

The temperature monitoring system 560 may provide sensor information tocontroller 550 in order to adjust at least one of a heating element, acooling element, a backside gas supply system, or an HV DC voltagesupply for an ESC before, during, or after processing.

Controller 550 includes a microprocessor, memory, and a digital I/O port(potentially including D/A and/or A/D converters) capable of generatingcontrol voltages sufficient to communicate and activate inputs tosubstrate holder 500 as well as monitor outputs from substrate holder500. As shown in FIG. 12, controller 550 can be coupled to and exchangeinformation with heating element control unit 532, cooling elementcontrol unit 522, HV DC voltage supply 534, backside gas supply system536, and temperature monitoring system 560. A program stored in thememory is utilized to interact with the aforementioned components ofsubstrate holder 500 according to a stored process recipe.

The controller 550 may also be implemented as a general purposecomputer, processor, digital signal processor, etc., which causes asubstrate holder to perform a portion or all of the processing steps ofthe invention in response to the controller 550 executing one or moresequences of one or more instructions contained in a computer readablemedium. The computer readable medium or memory is configured to holdinstructions programmed according to the teachings of the invention andcan contain data structures, tables, records, or other data describedherein. Examples of computer readable media are compact discs, harddisks, floppy disks, tape, magneto-optical disks, PROMs (EPROM, EEPROM,flash EPROM), DRAM, SRAM, SDRAM, or any other magnetic medium, compactdiscs (e.g., CD-ROM), or any other optical medium, punch cards, papertape, or other physical medium with patterns of holes, a carrier wave,or any other medium from which a computer can read.

Controller 550 may be locally located relative to the substrate holder500, or it may be remotely located relative to the substrate holder 500via an internet or intranet. Thus, controller 550 can exchange data withthe substrate holder 500 using at least one of a direct connection, anintranet, or the internet. Controller 550 may be coupled to an intranetat a customer site (i.e., a device maker, etc.), or coupled to anintranet at a vendor site (i.e., an equipment manufacturer).Furthermore, another computer (i.e., controller, server, etc.) canaccess controller 550 to exchange data via at least one of a directconnection, an intranet, or the internet.

Optionally, substrate holder 500 can include an electrode through whichRF power is coupled to plasma in a processing region above substrate510. For example, support base 520 can be electrically biased at an RFvoltage via the transmission of RF power from an RF generator through animpedance match network to substrate holder 500. The RF bias can serveto heat electrons to form and maintain plasma, or bias substrate 510 inorder to control ion energy incident on substrate 510, or both. In thisconfiguration, the system can operate as a reactive ion etch (RIE)reactor, where the chamber and upper gas injection electrode serve asground surfaces. A typical frequency for the RF bias can range from 1MHz to 100 MHz and is preferably 13.56 MHz.

Alternately, RF power can be applied to the substrate holder electrodeat multiple frequencies. Furthermore, an impedance match network canserve to maximize the transfer of RF power to plasma in the processingchamber by minimizing the reflected power. Various match networktopologies (e.g., L-type, pi-type, T-type, etc.) and automatic controlmethods can be utilized.

Additional details for the design of a temperature controlled substrateholder configured for rapid and uniform control of substrate temperatureare provided in U.S. Patent Application Publication No. 2008/0083723;U.S. Patent Application Publication No. 2010/0078424; U.S. PatentApplication Publication No. 2008/0083724; U.S. Patent ApplicationPublication No. 2008/0073335; U.S. Pat. No. 7,297,894; U.S. Pat. No.7,557,328; and U.S. Patent Application Publication No. 2009/0266809.

In one embodiment, the plasma etch process may comprise a processparameter space that includes: a chamber pressure ranging up to about1000 mTorr (milli-Torr) (e.g., up to about 200 mTorr, or up to about 30to 60 mTorr), a process gas flow rate ranging up to about 2000 sccm(standard cubic centimeters per minute) (e.g., up to about 1000 sccm, orabout 1 sccm to about 100 sccm, or about 10 sccm to about 50 sccm, orabout 30 sccm), an additive gas flow rate ranging up to about 2000 sccm(e.g., up to about 1000 sccm, or about 1 sccm to about 20 sccm, or about5 sccm), an upper electrode (e.g., element 70 in FIG. 6) RF bias rangingup to about 2000 W (watts) (e.g., up to about 1000 W, or up to about 500W), and a lower electrode (e.g., element 22 in FIG. 6) RF bias rangingup to about 1000 W (e.g., up to about 500 W). Also, the upper electrodebias frequency can range from about 0.1 MHz to about 200 MHz, e.g.,about 60 MHz. In addition, the lower electrode bias frequency can rangefrom about 0.1 MHz to about 100 MHz, e.g., about 2 MHz.

In another alternate embodiment, RF power is supplied to the upperelectrode and not the lower electrode. In another alternate embodiment,RF power is supplied to the lower electrode and not the upper electrode.In alternate embodiments. RF power and/or DC power may be coupled in anyof the manners described in FIGS. 4 through 11.

The time duration to perform a plasma etch process may be determinedusing design of experiment (DOE) techniques or prior experience;however, it may also be determined using endpoint detection. Onepossible method of endpoint detection is to monitor a portion of theemitted light spectrum from the plasma region that indicates when achange in plasma chemistry occurs due to change or substantially nearcompletion of the removal of a particular material layer from thesubstrate and contact with the underlying thin film. After emissionlevels corresponding to the monitored wavelengths cross a specifiedthreshold (e.g., drop to substantially zero, drop below a particularlevel, or increase above a particular level), an endpoint can beconsidered to be reached. Various wavelengths, specific to the etchchemistry being used and the material layer being etched, may be used.Furthermore, the etch time can be extended to include a period ofover-etch, wherein the over-etch period constitutes a fraction (i.e., 1to 100%) of the time between initiation of the etch process and the timeassociated with endpoint detection.

The plasma etch process described above may be performed utilizing aplasma etching system such as the one described in FIGS. 4 through 11.Furthermore, the plasma etch process described above may be performedutilizing a temperature controlled substrate holder in a plasma etchingsystem such as the one described in FIG. 12. However, the methodsdiscussed are not to be limited in scope by this exemplary presentation.

As noted above, the present inventors discovered that using a balance ofprocess gas containing C, H and F, and an additive gas containing CO₂may achieve an anisotropic etch profile with reduced silicon recess.FIG. 13 provides a schematic illustration of a film stack 300 to bepatterned using the plasma etch process described above. The film stack300 comprises substrate 310, high aspect ratio process (HARP) oxidelayer 305, silicon nitride layer 320, un-doped silicate glass (USG)layer 330, organic dielectric layer (ODL) 340, silicon-containinganti-reflective coating (SiARC) layer 345, and patterned photo-resistlayer 350. The patterned photo-resist layer 350 may be prepared usingconventional lithographic techniques known to those skilled in the artof pattern etching.

TABLE 1 He Pressure Etch Process UEL RF LEL RF p T (° C.) (LEL- (Torr)(LEL- CF₄ CHF₃ CO₂ Ar O₂ time Number (W) (W) (mTorr) C, LEL-E) C, LEL-E)(sccm) (sccm) (sccm) (sccm) (sccm) (sec) 1 600 100 40 40, 25 20, 30 5050 5 180 2 250 300 40 60, 60 20, 30 28 1500 180 3 250 300 40 60, 60 20,30 28 5 1500 180

The layers in film stack 300 are patterned using a series of etchingsteps. Table 1 provides three exemplary process conditions forpatterning the silicon nitride layer 320. For each plasma etch process,a process condition is recited including a process no., an upperelectrode (UEL) power (watts, W), a lower electrode (LEL) power (watts,W), a gas pressure (milli-Torr, mTorr) in the plasma etching system, atemperature set for components in the plasma etching system (° C.)(“LEL”=Lower electrode temperature, i.e., substrate temperature, at thecenter “LEL-C” and edge “LEL-E”), a CF₄ flow rate (standard cubiccentimeters per minute, sccm), an CHF₃ flow rate, a CO₂ flow rate, an Arflow rate, an O₂ flow rate, and etch time (sec, seconds).

TABLE 2 Center Edge Center Edge Process recess recess profile profileNumber (nm) (nm) (degrees) (degrees) 1 14.3 13.8 88.7 88.9 2 4.8 4.886.3 87 3 7.1 7.1 89.5 89

FIGS. 14A through 14D provide SEM (scanning electron microscope)photographs of the patterned silicon nitride layer and an exploded viewof the recess at the center and the edge of the substrate, respectively,for process number 1 (CF₄/CHF₃/O₂ plasma chemistry). FIGS. 15A and 15Bprovide SEM photographs of the patterned silicon nitride layer at thecenter and the edge of the substrate, respectively, for process number 2(CHF₃/Ar plasma chemistry). FIGS. 16A and 16B provide SEM photographs ofthe patterned silicon nitride layer at the center and the edge of thesubstrate, respectively, for process number 3 (CHF₃/Ar/CO₂ plasmachemistry). Table 2 summarizes the results for the three plasma etchprocesses and provides the silicon recess (nanometers, m) and thesidewall angle.

As evident in FIGS. 14 through 16 and Table 2, the use of CHF₃ alonewith a noble gas improves the silicon recess, i.e., reduces the siliconrecess; however, the etch anisotropy deteriorates, i.e., sidewall angledecreases from 90 degrees, or angular deviation increases. When CO₂ isintroduced in process number 3 to the CHF₃ only condition, the etchanisotropy is improved, i.e., commensurate with that of process number1, while at the expense of silicon recess. Yet, the silicon recess isreduced from process number 1.

Although only certain embodiments of this invention have been describedin detail above, those skilled in the art will readily appreciate thatmany modifications are possible in the embodiments without materiallydeparting from the novel teachings and advantages of this invention. Forexample, although one exemplary process flow is provided for preparing aspacer structure, other process flows are contemplated. Accordingly, allsuch modifications are intended to be included within the scope of thisinvention.

1. A method for selectively etching a substrate, comprising: disposing asubstrate comprising a silicon nitride (SiN_(y)) layer overlying siliconin a plasma etching system; transferring a pattern to said siliconnitride layer using a plasma etch process, wherein said plasma etchprocess utilizes a process gas composition having as incipientingredients a process gas containing C, H and F, and an additive gasincluding CO₂; and selecting an amount of said additive gas in saidplasma etch process to achieve: (1) a silicon recess formed in saidsilicon having a depth less than 10 nanometers (nm), and (2) a sidewallprofile in said pattern having an angular deviation from 90 degrees lessthan 2 degrees.
 2. The method of claim 1, further comprising: extendingsaid plasma etch process by an over-etch period of time to uniformlytransfer said pattern to said silicon nitride layer on said substrate.3. The method of claim 1, wherein said process gas containing C, H and Fcomprises CHF₃, CH₃F, or CH₂F₂, or any combination of two or morethereof.
 4. The method of claim 1, wherein said process gas containingC, H and F consists of CHF₃.
 5. The method of claim 1, wherein saidprocess gas composition further comprises a noble gas.
 6. The method ofclaim 1, wherein said process gas composition further comprises anoxygen containing gas, said oxygen containing gas comprises oxygen (O₂),CO, NO, N₂O, or NO₂, or any combination thereof.
 7. The method of claim1, wherein said process gas composition consists of CHF₃, CO₂, and Ar.8. The method of claim 1, wherein said silicon recess is less than 8 nm.9. The method of claim 1, wherein said angular deviation is less than 1degree.
 10. The method of claim 1, wherein said plasma etch processincludes setting a pressure in said plasma etching system ranging from30 mTorr to 60 mTorr.
 11. The method of claim 1, wherein a flow rate ofsaid process gas containing C, H and F ranges from about 10 sccm toabout 50 sccm.
 12. The method of claim 1, wherein a flow rate of saidadditive gas ranges from about 1 sccm to about 10 sccm.
 13. The methodof claim 1, wherein said plasma etch process includes coupling firstradio frequency (RF) power at a first frequency to a substrate holderupon which said substrate rests in said plasma etching system, andcoupling second radio frequency (RF) power at a second frequency to anupper electrode opposing said substrate on said substrate holder. 14.The method of claim 13, wherein said first radio frequency (RF) powerranges up to 500 W, and said second radio frequency (RF) power ranges upto 500 W.
 15. The method of claim 1, wherein a ratio between a flow rateof said additive gas and a flow rate of said process gas containing C, Hand F ranges from 5% to 25%.
 16. The method of claim 1, wherein a ratiobetween a flow rate of said additive gas and a flow rate of said processgas containing C, H and F ranges from 15% to 20%.
 17. The method ofclaim 1, wherein said process gas containing C, H and F consists ofCHF₃, and wherein a ratio between a flow rate of said CO₂ and a flowrate of said CHF₃ ranges from 15% to 20%.
 18. The method of claim 1,wherein said substrate rest on a temperature controlled substrate holderin said plasma etching system, said temperature controlled substrateholder comprises: a support base having fluid channels to circulate atemperature controlled thermal fluid in said support base; and asubstrate support coupled via a thermal insulator to an upper portion ofsaid support base, said substrate support comprising: one or moreheating elements embedded within said substrate support, an uppersurface to support said substrate by contact between said upper surfaceand a backside of said substrate, and an electrostatic clamp electrodeto hold said substrate on said upper surface of said substrate support.19. The method of claim 18, wherein said temperature controlledsubstrate holder further comprises: a backside gas supply systemconfigured to supply a heat transfer gas to the backside of saidsubstrate through at least one of a plurality of orifices or channelsdisposed on said upper surface of said substrate support, wherein saidplurality of orifices of said backside gas supply system are arranged ina plurality of zones on said upper surface of said substrate support tovary a backside pressure in a radial direction between a substantiallycentral region of the backside of said substrate and a substantiallyedge region of the backside of said substrate.
 20. The method of claim1, wherein said silicon nitride layer is part of an insulation spacerfor a transistor gate.